Typical semiconductor lasers such as laser diodes are formed by a body of semiconductor material having a thin, active region formed between cladding layers and contact regions of opposite polarity. A waveguide is formed in the structure by defining a stripe for light guiding and for current injection. Light is generated in the active region when the stripe region is subject to a current flow between the positive and negative contact regions. Cladding and confinement regions, among others, are placed between the contacts and the active region for guiding and confining the light along the thickness of the layers. The various regions typically are formed as substantially parallel thin layers grown epitaxially. When the current is greater than the threshold current for the active waveguide, amplified light is generated. In general, the greater the current flowing into the active waveguide, the more light is generated.
The active regions can be shaped like a thin layer having a specified thickness, length and width. The oscillation of the electric and magnetic fields of the light waves are restricted to specific modes, depending on the dimensions of the active layer. The longitudinal mode of the light, along the direction of propagation, is determined by the longitudinal length of the active layer forming the laser cavity. Similarly, the thickness of the active layer restricts oscillation of the light waves in the transverse direction, perpendicular to the plane of the layers along which light propagates. By appropriately sizing the thickness of the layer, oscillations can be restricted to the fundamental mode or to other desired modes of the light.
However, in the lateral direction perpendicular to the length of the cavity and in the same plane as the layers, the modes are not limited by the size of the active layer, but rather by the width of the stripe and of the current flow region. More than one mode can thus co-exist simultaneously within the active layer.
One problem encountered in this type of semiconductor laser diode is that the light emitted may include more than one optical mode in the lateral direction, as described above. The multi mode light emitted from this type of diode is thus of limited use, because it is formed by a complex pattern of bright and dark areas. Many applications require laser light that has a far field pattern consisting of a single bright spot, made, for example, by light that includes only the fundamental mode. For other uses, a different specific pattern can be required, such as one that is achieved by generating light having various selected modes.
A conventional method used to control the lateral modes of the light includes forming a positive conductor on top of the semiconductor layer, having a lateral dimension selected to only support the desired modes. An insulator can be placed between the active layer and the positive conductor layer outside of that lateral dimension, to prevent current flowing from the positive conductor outside of the selected region.
This method works for low power applications, but when the gain current flowing from the positive to the negative conductor and across the active layer exceeds a certain value, the current tends to spread in the lateral direction as it travels perpendicular to the layers. The degree of lateral current spreading can also increase with increased gain or drive current levels. This forms areas of high gain in the active layer that are larger than what is necessary to support the selected modes. When this occurs, extraneous modes can be supported by these enlarged gain areas of the active layer, and the light emitted is no longer of only the desired mode.
Accordingly, there is a need for a device and a method for controlling the lateral spread of current through an active layer of a semiconductor laser diode, so that the gain regions of the active layer can be limited in the lateral direction to only support desired lateral modes of the generated laser light, and in particular to support only the fundamental mode of the laser light.
The present invention is directed to a semiconductor laser diode and a related method that is adapted to control the lateral modes of the laser light generated, so that only desired modes are supported. In particular, this result is achieved by controlling the lateral spread of the electric current that passes through the active layer, so that only a selected portion of the active layer has a high gain, resulting in amplification of only the light crossing that portion of the layer. The other portions of the active layer that flank the selected active region inhibit the flow of current, and therefore have a lower gain which results in less amplification, or no amplification of the light passing through those portions. The lateral dimensions of the high gain portion of the active layer can be selected to support only desired modes of the laser light, such as the fundamental mode or a combination of the fundamental and other modes.
The lateral control of the electric current is achieved by implanting high energy ions, such as protons, in the portions of the active layer that require a reduced conductivity, while shielding from the ion implant the portion of the active layer where high conductivity, therefore high gain is desired. This shielding can be obtained, for example, by placing a photoresist layer between the source of ions and the active layer. The photoresist layer can be shaped with openings that correspond to the size of the desired conductive portion of the active layer.
To achieve these and other advantages and in accordance with the purpose of the invention as embodied and broadly described, in one aspect the invention is a ridge waveguide semiconductor laser diode adapted to support desired lateral modes of generated light, comprising a first conductor layer for application of a current, a second conductor layer facing the first conductor layer, an active layer disposed between the first and second conductor layers, a conduction region of the active layer adapted for conducting the current, and reduced conductivity regions of the active layer, flanking the conduction region, adapted to impede passage of the current.